chore(06-02): download and include Action Scheduler library

- Downloaded Action Scheduler 3.9.3 from GitHub
- Placed in vendor/action-scheduler/ directory
- Included in main plugin file before other code for proper initialization
- Library will auto-initialize itself when required

vendor/action-scheduler/classes/ActionScheduler_NullLogEntry.php

@@ -0,0 +1,18 @@
+<?php
+
+/**
+ * Class ActionScheduler_NullLogEntry
+ */
+class ActionScheduler_NullLogEntry extends ActionScheduler_LogEntry {
+
+	/**
+	 * Construct.
+	 *
+	 * @param string $action_id Action ID.
+	 * @param string $message   Log entry.
+	 */
+	public function __construct( $action_id = '', $message = '' ) {
+		// nothing to see here.
+	}
+
+}